First and foremost, bleeding during pregnancy can be dangerous so you should call your physician if you have any signs of vaginal bleeding during pregnancy.
Vaginal bleeding can be somewhat common in the first trimester. This can present as anything from light spotting to sever bleeding with clots. First trimester bleeding complicates between 20 and 30 percent of all pregnancies. Up to half of those who experience this may go on to have a miscarriage. Early bleeding may be a sign of an ectopic pregnancy. An ectopic pregnancy is one in which the fetus is not inside the uterus. An ectopic pregnancy may threaten the life of the mother. First trimester bleeding may also be implantation bleeding (a small amount of spotting caused by the embryo implanting into the uterine wall). It can be a threatened, completed or incompleted miscarriage. It can be caused by a blighted ovum. A molar pregnancy (in which the fetus is not actually a baby but an abnormal tissue) can cause bleeding. Postcoital bleeding may be common during pregnancy as well.
Any vaginal bleeding during the last 6 months of a 9-month pregnancy is considered abnormal. Most late-pregnancy bleeding is caused by a problem with the placenta. A placentia previa occurs when the placenta completely covers the opening of your womb (the cervix) and some of the blood vessels of the placenta stretch and rupture. A placental abruption occurs when a normal placenta separates from the uterine wall prematurely and blood pools between the placenta and the uterus. A uterine rupture is a splitting open of the uterus and may also cause late-pregnancy bleeding. In rare cases late-pregnancy bleeding may be caused by a fetal vessel rupture, in which the vessels from the umbilical cord attach to the membranes instead of the placenta. Other even more rare causes can be injuries or lesions of the cervix and vagina, including polyps, cancer and varicose veins.
Bleeding after the 28th week of pregnancy is a true emergency. This bleeding can be very mild to extremely brisk, and may or may not have accompanying abdominal pain. Hemorrhage is the most common cause of death of the mother in the US, and complicates roughly 4 percent of pregnancies.